In surface micromechanical pressure sensors, an active, i.e. sensitive, chip side is exposed to a medium the pressure of which is to be measured. Moisture present in the medium as well as aggressive constituents, such as acids, lyes, oxidizing media, salts, and organic solvents, may attack the chip surface and render the sensor completely or partly non-functional. This is a problem especially in the automobile area, where such sensors have to function within tight tolerance boundaries for years.
Existing solutions count on the protection of the sensor surface by a combination of normal chip passivation (e.g. silicon nitride) and mechanical protection of a low-viscous gel. But since the gel is permeable for gasses and partly also for ions, this solution has disadvantages. Humidity may diffuse through the gel in unhindered manner. If there are places of bad adhesion or contaminations between gel and chip surface, moisture may condense there and lead to surface currents or to a change of the dielectric constant. Particularly in voids in the passivation, this may lead to complete failure of the device. A further problem is the solubility of gasses in the gel. At pressure release, this leads to a sudden formation of bubbles in the gel (sparkling effect). By these bubbles, the gel may tear open and the protective function is lost. Moreover, offset errors may arise in the pressure signal. There is not yet a satisfactory solution for the bubble formation at pressure changes. This problem of bubble formation in gels, however, only occurs in applications with pressures greater than 3 bar (e.g. in the tire pressure measurement). Furthermore, gel is not environment-resistant, for example with reference to organic solvents such as gasoline, whereby the gel then dissolves for example on contact with such organic solvents. A pressure sensor or protection of a pressure sensor using gel can thus, under certain circumstances, not be used in the automotive industry, since pressure sensors to be applied in the automobile area have to withstand extreme environmental conditions. Among these extreme environmental conditions are for example high temperature variations (e.g. of a variation range of about 80° C. between summer and winter) as well as compatibility with substances acting organically as solvents, such as gasoline or similar fuels.
Another existing solution for the protection of the active chip side or the sensor surface, which obviates the above-mentioned problems, is the use of a housing with a metal membrane and an oil filling. The flexibility of the metal membrane is achieved by embossed, concentric grooves. This solution, however, is technically intensive and expensive and therefore not suited for mass production. In particular, this results from such an oil-filled pressure sensor container having to be produced from a special material (such as stainless steel), in order to prevent deformation of the container at temperature-induced expansion of the oil and thus possible arising leakage of such an oil-filled pressure sensor housing. Moreover, an oil-adapted membrane for balancing the (e.g. temperature-induced) oil pressure change and thus for compensating for possible offset-bearing measurement results has to be welded onto the oil-filled housing in a first weld with the housing. Furthermore, the oil has to be filled in a state in which the housing is already provided with the membrane, and the (stainless steel) housing has to comprise special capillary roles to enable the oil to be filled in the housing uniformly and without air inclusions. Furthermore, this requires evacuating the housing to again minimize the probability of air inclusions occurring in the oil filling, since such air inclusions in the oil lead to flawed pressure transfer between the membrane and the pressure sensor due to the compressibility of gasses. Furthermore, the housing has to be welded a second time when the oil has been filled in, whereby such an oil-filled pressure sensor housing proves cost-intensive due to the materials to be used and the production steps necessary for fabrication (with two weld connections).